This invention relates to supercharged two-stroke internal combustion engines and particularly, although not exclusively, to engines of this type ignited by compression or Diesel engines.
Two-stroke engines require scavenging means for removing the burned gases in the combustion chamber or chambers.
One of the scavenging systems currently in use, which may be described as a system of scavenging "from below" makes use of intake and exhaust ports in the cylinder wall, which are opened by the piston when the latter is close to its lower dead centre. The exhaust ports are generally longer than the intake ports so that they are the first to be uncovered when the piston approaches its lower dead centre. This solution requires a high stroke/bore ratio for the engine so that the ports offer a sufficient passage to the gases. In practice, such an engine has a stroke/bore ratio of at least 1.2. The useful stroke (during which the cylinder is separated from the intake and exhaust pipes) may be, for example, about 0.6 time the total piston stroke in the case of high speed engines.
Another known scavenging system, which may be described as "unicurrent", comprises intake ports and one or more exhaust valves. The "permeability" of the ports in this case increases with the stroke/bore ratio while the "permeability" of the valves decreases as this ratio increases. A compromise solution is therefore arrived at, amounting to a stroke/bore ratio of about 1. The useful stroke is higher than in the preceding case, since there is no exhaust port. It is generally about 0.8 time the total stroke. The scavenging phase, during which the intake ports are open, generally extends over an angle of rotation of the crankshaft of 120.degree. about the lower dead centre.
Both solutions have the disadvantage of requiring a relatively high stroke/bore ratio. At the most, the engine may be "square" (i.e. with a stroke/bore ratio equal to unity). An analysis of the structure and operation of two-stroke engines shows that it would be advantageous to reduce the stroke/bore ratio to the lowest possible value, that is to say to design a "super-square" engine. In particular, at a given linear piston velocity, a super-square engine runs faster and develops more power than another engine of the same cubic capacity. Two-stroke engines having a conventional scavenging system are practically impossible to design as super-square engines because of the length of dead stroke required for closing the ports.
In addition, using ports for scavenging results in complexity of the cylinder jacket. Moreover, the passage of the piston rings in front of the ports makes design of the rings difficult. Lastly, it is necessary to provide one or more additional packing rings at the bottom of the piston skirt to ensure a tight seal between the crank case and the intake manifold. This results in a longer and therefore heavier piston and a longer and more bulky cylinder sleeve.
Any reduction in the stroke/bore ratio would also enable the diameter of the crank case to be reduced, due to the shorter piston stroke for a given cubic capacity.
Another disadvantage resides in the difficulty of lubricating a cylinder jacket formed with apertures without excessive consumption of oil and deposit of carbon at the exhaust ports.
In another prior art two-stroke engine (German Pat. No. 905,687), there is provided an exhaust valve located in the cylinder head and a lateral intake valve: in such an engine, satisfactory scavenging cannot be achieved.
It is an object of the invention to provide a supercharged two-stroke engine in which the limitations mentioned above are considerably attenuated, and which is appropriately scavenged even if super-square.
According to one aspect of the invention, there is provided an engine having at least a cylinder block and a cylinder head cooperating with at least one piston reciprocably received in a cylinder formed in said cylinder block to define at least one expansible combustion chamber, at least one intake valve slidably received in said cylinder head and cooperating with an intake valve seat to control airflow from an intake pipe into said chamber; at least one exhaust valve slidably received in said cylinder head and cooperating with an exhaust valve seat to control gas flow from said combustion chamber to an exhaust pipe; and means for operating said intake and exhaust valves in proper time sequence with the displacement of said piston, whereby both said valves are open while the piston is at its bottom dead centre, said intake valve, intake pipe and cylinder head being constructed for air to be directed toward the piston and substantially parallel to the wall of said cylinder upon opening of the intake valve and to scavenge the chamber without substantial direct flow of air from the intake pipe to the exhaust pipe along the cylinder head.
The intake pipe (and generally preferably the exhaust pipe as well) is typically directed substantially parallel to the direction of movement of the piston, at least in a portion thereof close to the seat of said intake valve and long enough to impress a direction of flow to the air admitted into the chamber.
In other words, the cylinder head is formed with a deflecting surface between said intake valve and exhaust valve for substantially preventing direct flow of air from the intake pipe to the exhaust pipe upon simultaneous opening of both valves and for directing the airflow admitted around said intake valve toward the piston along the cylinder wall.
The valves are advantageously disposed symmetrically in the cylinder head. In particular, the valves may be directed at an angle of about 45.degree. from the axis of the chamber, the valve heads or disks moving to and from valve seats formed at the bottom of recesses in the cylinder head. The intake and exhaust pipes are then advantageously positioned in the direction of the flow of gas. To improve the flow pattern, the end face of the piston may be recessed and have a part spherical cavity.
A stroke/bore ratio of about 0.9 can in this way be obtained without difficulty. Since scavenging takes place with the piston near the lowest dead centre, when the piston is far from the valves, the valves may have a considerable opening stroke so that a high "permeability" is obtained; moreover, this permeability increases with the angle of inclination of the valves. With four valves arranged at 45.degree. in a roof-shaped cylinder head, a stroke/bore ratio of 0.9 and a useful stroke amounting to 0.8 time the total stroke, the cross-sectional area available for the passage of the scavenging air and gases is substantially equivalent to that which can be obtained with "scavenging from below" through cylinder ports in an engine having a stroke/bore ratio of 1.2 and a working stroke amounting to 0.62 time the total stroke. The cross-sectional area will however be smaller than that available for "unicurrent" scavenging through ports and valves. This limitation is acceptable when a supercharging system enables very high levels of supercharging to be obtained, as will be explained hereinafter, and/or when the valves are provided with a fast action operating mechanism. Then, the reduction in the volumetric filling coefficient is balanced by the fact that a higher air pressure is established in the cylinders and/or the time during which the valves are partially opened is decreased.
The invention enables a conventional four-stroke engine to be converted into a supercharged two-stroke engine with a substantial power increase. All that is necessary is to change the cylinder head, the valves and the distributor mechanism and to machine the pistons. This type of conversion is of particular interest in the case of a "super-square" four-stroke engine.
It has already been proposed to maintain a moderate pressure in the crank case of a piston engine. The advantage of such a pressure is quite limited, even if different surfaces are subjected to the pressures in the combustion chamber and the crank case (German Pat. No. 737,206).
According to another aspect of the invention, there is provided a super-charged two-stroke engine, in particular a Diesel engine, comprising at least one cylinder and at least one piston reciprocably moving therein, the piston and cylinder together forming a combustion chamber of variable volume, in which engine the piston and crank case together delimit a compartment which is substantially gas tight and the volume of which does not substantially change, means being provided to maintain a pressure amounting to between one quater and one half the pressure produced in the combustion chamber in the course of one operating cycle when the engine is under full load in said compartment when the engine is running.
This pressurization of the crank case eases the load on the moving connecting parts and enables the engine to tolerate very high combustion pressures, amounting to about twice the pressure which could be accepted in a conventional Diesel engine (pressure of the order of 250 bar). By this means, it is possible to derive the maximum possible benefit from the supercharging pressures obtainable with the supercharging system to be described hereinafter, and to increase proportionately the power developed by the engine.
Pressurization of the crank case is more easily achieved in an engine with valves of the kind defined above, mainly because such an engine has a much smaller crank case than a conventional supercharged two-stroke engine and--for a given internal pressure--the wall thickness of the case increases in proportion to its diameter. At given thickness, therefore, much higher pressures can be sustained in a smaller crank case such as the one used in a two-stroke engine which is super-square. For similar reasons, this arrangement eliminates the excessive consumption of oil, which would otherwise be carried out through the ports by the air flow.
In a two-stroke engine with conventional scavenging means, the ports constitute a zone of low pressure. Packing of the piston skirt with rings between the intake manifold and the crank case must be extremely efficient. If the case is pressurized at 75 bar, for example, the escape of air from the case through an effective cross-sectional area of 1 mm.sup.2 represents a loss of power of 10 hp. In the case of a two-stroke engine with valves of the kind defined above, there is no such low pressure zone and sealing of the bottom of the piston skirt with packing rings is then not necessary. The leaks from the crank case towards the combustion chamber when the piston is close to the lower dead centre is more or less balanced by leaks from the chamber towards the crank case when the piston is close to the upper dead centre.
The invention will be better understood from a consideration of the following description of a supercharged two-stroke engine which constitutes a particular embodiment of the invention given by way of non-limiting example. The description refers to the accompanying drawings.